1. Field of the Invention
The present invention generally relates to a device for sensing the temperature of a high temperature and high pressure fluid, such as a reactor coolant fluid in the piping of a nuclear reactor system and, more particularly, to such a dual temperature sensing device having a twin well thermowell for accommodating dual resistance temperature detectors.
2. State of the Relevant Art
Temperature sensing devices are well known and have been used for a variety of purposes, including measuring the temperature of a reactor coolant flowing through conduits including, variously, the hot and cold legs of the system piping of a nuclear reactor system. As explained in more detail hereafter, the phenomenon known as temperature streaming may produce a large temperature gradient in the cross-section of the fluid passing through the piping. This introduces the requirement of obtaining multiple temperature measurements and deriving an average value thereof. As a typical, or practical, example, the temperature measurements are obtained at three (3) locations equiangularly displaced about the circumference of the piping; the respective, measured values at the three locations then are averaged.
Recently, there has arisen a need for an increased number of temperature measurements in this particular environment. The need for deriving an increased number of temperature measurements, however, conflicts with the need to minimize or avoid additional piping penetrations. More particularly, the need to obtain a greater number of temperature measurements arises out of the increasingly stringent regulations regarding adequate safeguards in the operation of nuclear power systems and particularly the requirement that independently derived temperature measurements be supplied to each of the control and protection systems for the reactor. As will be readily appreciated by those of skill in the art, this requirement relates to the ability of the control and protection systems of a nuclear power plant to generate a trip signal in the event that a malfunction is detected as a result of the temperature indication exceeding certain predetermined limits of safe operation. Accordingly, the requirement differs, as between the types of plants.
For example, in a four (4) loop plant, having four (4) sets of control and protection systems and employing a two out of four (2/4) logic, a protection function, such as reactor shut-down, will be initiated if two of the four control and protection systems function to generate a trip signal. Thus, one such control and protection system may be out of service and another (i.e., a second such system) may fail and there will still remain two systems fully functional for generating a trip signal. In each of these four (4) such systems, the temperature indication may be supplied to the protection system first and subsequently through an isolator to the control system.
In the case of a three (3) loop plant, however, employing two out of three (2/3) logic, it is necessary to separate the temperature inputs to the protection and control systems since, by comparison to and adopting the same rationale as discussed above regarding four loop plants, assurances must be provided that the failure of the control system will not also produce failure of the associated protection system. As thus will be apparent, if one such control and protection system is out of service and another should fail, the required inputs (i.e., detected temperature conditions) to satisfy the 2/3 logic are not satisfied. Thus, to meet the more stringent requirements as aforestated, the three loop system requires independently derived temperature indications for each of the protection and control systems of each loop. In the case of two (2) loop plants employing two out of four (2/4) logic, and analogously to the four loop plants, two sets of independent temperature measurements must be derived for each of the two loops and thus for their corresponding control and protection systems.
It is also a requirement that substantially the identical temperature measurements be produced and separately supplied to the control and protection systems, to avoid discrepancies in the operations of the control and protection systems. Because of the temperature streaming phenomenon, deriving two separate and independent average temperature values, each average being obtained from three (3) locations, implies the use of six (6) such independent temperature sensing devices, such as resistance element temperature detectors, and correspondingly the provision of six (6) penetrations of the piping for accommodating the six (6) sensing devices. This, however, conflicts with the requirement of minimizing or avoiding additional piping penetrations. Moreover, due to the temperature streaming phenomenon, produced by the two sets of sensing devices it is difficult, if not impossible, to physically locate the six separate sensing devices and their corresponding penetrations of the piping such that the two average temperature temperatures, each derived from a corresponding set of three (3) independent sensing devices, will be of identical values. As discussed in the following, the prior art simply has not addressed the problems which are solved by the present invention.
Single detector element thermowell devices are well known in the art and one such device is illustrated, for example, in U.S. Pat. No. 4,510,343. The general proposition of mounting two or more thermocouples within a common casing has long been recognized in the art, as evidenced by U.S. Pat. No. 721,770. Thus, the number of temperature measurements may be increased without increasing the number of piping penetrations, simply by installing a dual element resistance temperature detector in a single thermowell. While this, at least in theory, satisfies the general objective of deriving an increased number of measurements, dual element resistance temperature detectors are subject to the common failure of both elements. Particularly in the environment of a nuclear reactor system, and as well in other applications in which highly reliable and accurate measurements of fluid temperature are required, the potential of common failure of both elements of such a dual element detector is unacceptable.
Numerous configurations of multi-point, or multiple element, thermocouple assemblies also have evolved and are known in the art. U.S. Pat. No. 4,075,036, for example, discloses the alternative structure, above-noted, of plural thermocouples disposed within a common shell or protective tube, and purportedly is suitable for use in the adverse environmental conditions of a nuclear system installation; as above-noted, however, the plural thermocouples are subject to common failure and thus such a device is unacceptable for the applications to which the present invention is directed. Additional examples of such structures may be found in U.S. Pat. No. 3,955,419 - Barton et al., assigned to the common assignee of the present invention as well as U.S. Pat. Nos. 4,028,139, 4,162,175, 4,385,197, and 4,410,756. In general, these patents disclose the provision of plural thermocouples disposed at varying locations within a common protective well or tube, and are designed to monitor temperatures at predetermined locations relative to the length of the protective tube and thus within the interior of a vessel or conduit within which the tube is inserted.
None of these prior art configurations, however, addresses the requirement to which the present invention is directed; in fact, the prior art structures have the contrary objective. More specifically, whereas the requirement is to obtain, simultaneously, two independent but substantially identical average temperature measurements at a common axial location, the structures of these above-referenced patents, instead, simultaneously and independently measure temperatures at respective, predetermined and different locations along the length of the protective tube--and necessarily produce temperature output readings of different values.
U.S. Pat. No. 4,186,605 discloses a set of thermocouples for measuring the average of several temperatures at predetermined positions about the interior circumference of a confining structure. The specific structure therein considered is the region of the high speed gas ring formed at the outside of a turbine, such as in a jet engine. As therein disclosed, plural probes, each containing one or more thermocouples, are placed at desired positions distributed in a circle in the gas ring. The number of probes generally is between two and twelve, depending upon the accuracy desired and the acceptable cost for a particular case. In the specific example illustrated, six probes are provided, each containing a thermocouple; the alternative is also suggested of employing several thermocouples in each probe, the hot wells of the thermocouples being located adjacent respective, different points along the length of the corresponding probe. This structure again does not satisfy the requirements to which the present invention is directed, since multiple penetrations are required for the multiple probes. Further, independent temperature measurements are derived from angularly displaced and/or both angularly and radially displaced positions about the interior of the conduit.
Duplex and multiple element thermocouple assemblies, on the other hand, also have been proposed in the prior art. One example thereof is disclosed in U.S. Pat. No. 4,217,463 and particularly comprises an assemblage of two small diameter thermocouples which are soldered into two axial channels which extend along the length of a stainless steel cylinder which forms a portion of a protective fitting for the duplex thermocouple. At the temperature sensing end of the assembly, the ends of the thermocouples project slightly beyond the end of the cylinder, affording a direct immersion-type configuration. The other end of the cylinder is soldered into a hole axially drilled into an end of a larger stainless steel cylinder, the latter having external screw threads for securing the assembly to the sidewall of a conduit or vessel, such as a polyethylene recctor. The opposite end of the larger cylinder extends exteriorly of the vessel and has an axial, partial bore therein in which a further fitting is pressed and soldered in place, and within which the thermocouple leads emerge from the protective sheaths for connection to external leads. The thermocouples are described as being soldered to the supporting structures for completing the sealed relationship. The structure thus disclosed affords dual measurements from substantially adjacent locations within a reactor or other conduit through which a fluid flows. However, the structure fails to satisfy the requirements of the present invention, since the thermocouples are permanently secured in position within the protective cylindrical support structures; hence, should a thermocouple fail, the entire duplex thermocouple assembly would necessarily require replacement, implying shut-down of the polyethylene reactor system with which it is disclosed for use. While a shut-down to replace temperature sensing thermocouples may be acceptable in the operation of a polyethylene reactor, it is highly impractical and unacceptable in the environment of a nuclear reactor, as those of skill in the art will readily appreciate.
Other forms of dual or multiple element temperature sensors are known in the art, such as are disclosed in U.S. Pat. Nos. 3,366,942, 3,898,638 and 4,448,943; these sensors, however, are utilized for the entirely different purpose of flow detection or liquid level detection. These devices employ a pair of related sensors at proximate but displaced positions, one of which is separately heated, for sensing a resultant temperature differential between the positions. Necessarily, such sensors are inherently unsuitable for the purpose of the present invention.